C Wire Connection issue on Wiring to Thermostat on Gas Steam Heat Boiler

5horizonsrr

Hi everyone,

I learned the hard way that despite them telling you otherwise, Nest thermostats need a "C" wire connection. Mine wasn't keeping a charge without it. The good news is the wiring harness in the wall already had one so install should be easy enough. However on the boiler end I have a bit of a conundrum. The wiring makes a stop at a an electrical junction/light as shown in the image below, then continues onwards to the boiler via the conduit pipe shown.

The problem is the blue (C) and Green wires stop at that 24V (step-down?) transformer instead of the boiler. Why I have no idea because they aren't connected. I'm guessing to give power to a long-departed thermostat. I need to run the blue wire further along to the boiler with the white and red, but i"m not sure where to connect it.

The second image is the back of the boiler electronics/transformer. The solder tabs don't line up how I would assume - (meaning that center blue wire I'm not sure is ground or not, and "C" has the white connected, I think to the pressultrol) so I need to sort where the thermostat "C" wire goes. Any ideas? Or what is a logical method to test this with a volt meter?

Thanks in advance!

Steamhead

That's wrong. Low-voltage thermostat wire should not be in the same conduit as line-voltage wire, unless it is insulated for the higher line voltage. Most thermostat wire is not.

It looks like the transformer by the light was used to operate an old mechanical clock thermostat. It is no longer needed, so I'd get rid of it and remove the thermostat wire from inside the conduit. Is the transformer in the second pic mounted on the boiler?

5horizonsrr

Thanks! The second image is mounted on the boiler. The opposite side is the on/off safety switch and a bunch of wire junctions for the pressuretrol, auto feed, thermostat R/W wires, and what looks like a power line in

Steamhead

Right. Is this a Weil-Mclain steam boiler?

5horizonsrr

Correct- Weil-McLain. Sadly I don't know what model.

Power into that boiler transformer is standard wall 120V AC.

On the back side as shown the small white wire on the C terminal goes to the ignitor/TH heat call terminal. Despite it being white, my multimeter confirms it to be ground. (and TR to be hot) The larger black wire also on the "C" goes to the automatic water feeder.

So knowing that, ok to connect the thermostat C wire to the C terminal? (and I will then swap the pilot leads to the accurate colors of course....)